The present invention is directed to a hollow annular drill bit with a hollow cylindrical carrier member with open recesses in its leading end in which cutter members are secured and project axially from the leading end of the carrier member and also outwardly from its inside and outside surfaces.
Hollow annular drill bits are used for forming large diameter boreholes in components, where the boreholes are used mainly for the passage of pipelines or the like. Masonry, concrete, rock or the like can be the materials forming the components.
The structure of hollow annular drill bits is well known and comprises, as a rule, a hollow cylindrical carrier member having a leading end with open recesses. Cutter members are secured in these open recesses and serve for the removal of material from the components being drilled. The cutter members can be formed of hard metal, polycrystalline diamond platelets, diamond cutting edges formed of diamond grains embedded in a matrix material, and the like. The connection between the cutter members and the carrier member can be effected by soldering, welding or sintering.
To assure that the material drilled from the components is carried away and that the friction of the carrier member within the borehole being formed does not become excessive, it has been known to form the annular drill bits so that an annular gap is produced between the carrier member and the borehole in the component. In such hollow annular drill bits it has been known to select the dimension of cutter members measured in the radial direction to be somewhat greater than the wall thickness of the carrier member, so that the cutter members project beyond the inner and outer surfaces of the carrier member. Such a hollow annular drill bit is disclosed in DE-OS 39 30 250.
Due to the difference between the wall thickness of the carrier member and the radial dimension of the cutter members, only a relatively small connection surface is available in such hollow annular drill bits for a soldered connection. Since hollow annular drill bits are subjected to considerable loads due to the torque applied and possibly also because of the forces acting on them, the cutter members can become detached and the hollow annular drill bit becomes useless.
In the hollow annular drill bit known in CH-PS 414 438 the entire leading end region of the carrier member is provided in the circumferential direction with a larger wall thickness which corresponds to the radial dimension of the cutter members As a result, there is no difference between the wall thickness of the carrier member and the radial dimension of the cutter members, so that the entire dimension of the cutter members in the radial direction is available for connection with the carrier member. The disadvantage of detachment of the cutter members is largely prevented, however, only with the substitution of a very essential disadvantage, namely that an increase in the friction of the carrier member within the borehole being produced is due to the larger wall thickness at the leading end region of the carrier member. Further, the drilled material cannot escape because there is no annular gap in this leading end region. Apart from the heat caused by friction, there is also a compaction of the drill material so that overloads are developed resulting in a drop in output which can lead to premature failure of the hollow annular drill bit.